1. Field of the Invention
The present invention relates to a 2-cycle engine provided with an exhaust control valve for varying a cross sectional area of an exhaust passage leading to an exhaust port, and particularly to a structure enabling logical provision of an exhaust control valve for a 2-cycle engine.
2. Description of Background Art
Two-cycle engines have been known as engines which are compact but can produce a high output because of their relatively simple structure and mechanism. Two-cycle engines have been widely applied to motorcycles, small boats, and the like.
In two-cycle engines, valves are disposed near the exhaust ports, and are opened or closed in order to control exhaust gases, thereby enabling the engine to operate under optimum conditions.
For instance, the two-cycle engine disclosed in Japanese Patent Laid-open Publication No. Hei 7-71279 or No. Hei 7-180556 includes exhaust control valves disposed on inner walls of exhaust passages near upper edges of exhaust ports in order to open or close the exhaust ports. When the engine is operating at a low speed or under a low load, the exhaust control valves substantially fully close the exhaust ports, optimally controlling pressure in the cylinders, activating fresh air in combustion chambers using thermal energy of already burnt gases remaining therein, realizing combustion in an active and hot atmosphere in which self-ignition combustion is carried out, and improving fuel economy and promoting purification of exhaust gases.
Japanese Patent Laid-open Publication No. Sho. 56-56915 exemplifies the two-cycle engine in which exhaust control valves for varying exhaust emission timing are positioned on inner walls of exhaust passages near upper edges of exhaust ports. When the engine is operating at a high speed, the exhaust control valves are fully opened to advance the timing for opening the exhaust ports (i.e., exhaust emission timing), thereby promoting discharge of exhaust gases. Conversely, when the engine is operating at a low speed, the exhaust control valves are nearly closed to retard the exhaust emission timing and prevent fresh air from blowing through the cylinders.
In the foregoing two-cycle engines, the exhaust control valves are supported by a pivot, are positioned near the upper edges of the exhaust ports, and are activated to vary the open areas of the exhaust ports.
Further, when the exhaust timing-is controlled, the edge of each exhaust control valve (i.e., the edge for opening or closing each exhaust port) is in contact with an inner surface of the exhaust port. This means that the pivot for supporting the exhaust control valves should be positioned as close as possible to the exhaust ports. If the pivot is positioned far from the exhaust ports, the exhaust control valves are subject to an increase in their turning radius, and suffer from reduced rigidity in their moving parts and an enlarged structure.
However, in the foregoing two-cycle engines, the exhaust control valves are directly supported by the cylinder block where the exhaust ports are formed. Therefore, in two-cycle engines, the cylinder block needs to be subjected to a complicated machining process for forming bearings and so on for receiving the pivot, so that it is difficult to attach the exhaust control valves to the cylinder block. Further, when the pivot is interposed between the cylinder block parts as in the last mentioned publication, the bearings should be precisely formed on surfaces of the cylinder block parts to be joined. This results in reduced efficiency in manufacturing engines.
Since the exhaust control valves are exposed to hot exhaust gases, the bearings and so on should have a cooling member. However, such a cooling member is difficult to add because structure for mounting the exhaust control valves is complicated.
In the case of an engine having multiple cylinders, there is a connection between control valves so as to drive the exhaust control valve of each cylinder in synchronism, but since each exhaust control valve is provided close to the exhaust port of each cylinder, there is a problem in that the connecting mechanism has a complicated structure. Furthermore, when the exhaust gas from each cylinder is concentrated by the exhaust manifold, the position where the exhaust gas is concentrated is separated from the exhaust port, because of a space needed for the exhaust control valve, which causes the problem of the exhaust manifold projecting from the cylinder block and the power unit being increased in size.